US2501702A - Vacuum gauge - Google Patents

Description

R. H. VARIAN March 28, 1950 VACUUM GAUGE Filed March 31. 1945 dej ml N. .WWE

INVENTOR RUSSEL/ H. l//w/F/N f ATTORNEY VPatented Mar. 28, 1950 UNITED STATES PATENT UFFICE VACUUM GAUGE Russell H. Varian, Garden City, N. Y., assigner to The Sperry Corporation, a corporation of Dela- Application March 31', 1945, Serial No. 586,006

9 claims. (ci. 175-183) The present invention relates to methods and apparatus for measuring very low gas pressures such as are encountered in evacuated electron discharge devices, usually referred to as vacuum tubes.

The principal object of the present invention is to measure the gas pressure within an evacuated space.

It is another object oi this invention to provide a method of determining the gas pressure in a vacuum tube. l

Another object is to provide -an improved vacuum gauge.

It is a further object to provide a vacuum gauge adapted for measuring gas pressures throughout a relatively extensive range such as the range from 10e to 10-10 millimeter of mercury.

In the present invention, electrons are projected along a directive beam through an evacuated space, and the compactness of the electron stream is modulated by the formation of ions therein. The number of ions within the electron beam tends to remain substantially equal to the number of electrons in the beam, and thus does not vary appreciably. yThis is seenfrom the fact at any instant in the beam, the beam tends to have a net positive charge which, by repelling the ions from the beam, increases the rate at l which theions drift from the beam. Thus, the

ion content of the beam is-self-ad-.iusting to a number of ions just suilicient 'to neutralize'the beam charge. The rate of formation ofvions vin the electronbeam by collision of electrons with gas molecules is approximately proportional to the gas pressure. Accordingly, as the gas pressure is decreased, and the rate roi? ion formation correspondingly decreased, each ion must remain effective in` the electron beam longer than under higher gas pressure conditions, in order to satisfy the above requirement that the number of ions within the electron beam remain substantially equal to the number of electrons short. Under a condition of very low gas pres-4 sure, new ions are formed infrequently, and the time span of effectiveness of an ion in the beam is very long.

If no ions were present within Y the space through which a beam of electrons is projected, the strong negative charge of the beam would tend to cause dispersion of the electrons. The presence of ions within this space tends to neutralize the negative charge, eliminating the electron-repellent tendencies and permitting a compact electron beam to be produced. The ions disappear from the beam and new ions are continually formed to replace them. Since the formation of a new ion is generally not simultaneous with the cessation of eilectiveness of another ion, it will be recognized that the number of ions eiective in the beam does not remain exactly uniform, but instead increases momentarily at the instant of each ion formation.

The amount of dispersion of the electrons from an ideally compact electron beam varies appreciably with a reduction or an increase by one ion in the number of ions effective in the electron beam. Accordingly, the compactness of the electron beam is modulated in accordance with the formation of new ions in the beam, the frequency of modulation being varied accordingly as the ion formation is frequent with high gas pressure, or infrequent with low gas pressure.

By providing an electron lcollector adapted to receive only those electrons which are within a predetermined very short distance from the axis y of the electron beam, there is produced an electric current which is modulatedv in accordance withthe compactness modulation of the electron beam. The modulationv of this current corresponding to a given gas pressure is not 'a simple periodic wave,since.new ions are' not formed in the electron beamv at precise, regular intervals. On the other hand. the modulation is characterized bv a distribution of energy through a wide range of frequencies with maximum amplitude at a frequency dependent upon the pressure of the gas. Due to inertia oi the ions, and to the tendency of all ions in the beam to be repelled when there is excess positive charge. very low- Thus, by analyzing the electron collector cur- 3 rent in terms of the frequency distribution of energy, the gas pressure in the evacuated space may be ascertained. Since the frequency distribution of energy varies as a predictable function of gas pressure throughout a range extending far beyond the range of gas pressures to which ordinary vacuumv gauges are limited, the present invention provides an absolute measurement of pressure in extremely high-vacuum devices.

Apparatus suitable for use in determination of the gas pressure within an evacuated system is illustrated in Fig. 1, while Fig. 2 illustrates the frequency distribution of output noise in the apparatus of Fig. 1 resulting from each of three different gas pressures Within the system.

Referring now to Fig. 1, an electron discharge device II, including a cylindrical metallic chamber I2, an electron gun I3 and an electron collector end I4, is connected as by a tubular conduit I5 in a system which may include a vacuum tube in the process of being evacuated.

The electron gun I3 comprises an insulating gas-tight supporting and enclosing portion such as a cup-shaped glass portion I6, which may be sealed to a tubular metal member I1 connected in turn to one end 36 of the cylindrical chamber I2. Within the glass portion I6 is provided an electron emitter or cathode I8 arranged symmetrically about the axis of the cylindrical chamber I2,

and supported by a rigid connection to a stiff :z:

terminal wire I9 extending through a seal 2| in the glass portion I6. A second conductor 22 similarly extending through the glass portion I6 may be connected to one end of a heater winding 23 having its opposite end connected to the wire I9.

A focusing electrode 24 is connected to vcathode I8 and supported in specially positioned relation therewith and arranged to direct electrons emitted by cathode I8 into a concentrated stream or electron beam along the axis of the cylindrical chamber I2, the cross-section of this stream becoming very small at the portion thereof approaching the end 31 of the cylindrical chamber I2.

The electron collector end I4 may include a tubular metal portion 3l connected to the end 31 of the chamber I2, and a cup-shaped glass portion.32 sealed to the tubular portion 3|. A collector electrode 33 is supported within the collector end by a further sti terminal wire 34, sealed as shown at 35 in the glass portion 32.

Each of the disc ends 36 and 31 of the cylindrical chamber I2 is provided with an opening symmetrical about the axis of the cylindrical chamber I2. The opening through the end 36 may be of a diameter substantially equal to or greater than the diameter of cathode I8 for permitting the electron beam produced by the cathode I8 and the focusing electrode 2i to enter the cylindrical chamber I2. The opening through the opposite end 31 of the chamber I2 preferably, but not necessarily, may be made appreciably smaller than the input opening, as illustrated in Fig. l, so that the relative extent to which the electron beam is directed through the exit opening in the end 31 will depend greatly upon thecompactness of the electron beam approaching this opening. If desired, an electron-permeable grid 38, which may comprise a screen of small metal wires or a perforated disc, for example, may be inserted within the opening in the end 36 of the chamber i2, and a similar grid 39 may be inserted within the electron exit opening in the end 31.

A rst electric energy source such as a battery III is connected through the terminal wires I9 and 22 to the heater winding 23 of the electron gun I3, for heating theY cathode I8 to provide electron emission therefrom.

An electron accelerating potential source such as a battery 42 is connected at its negative terminal to the cathode I8 and at its positive terminal to the chamberV I2 for accelerating to an apreciable velocity thev electrons in the beam issuing from the electron gun I3. The source 42 may provide an accelerating potential of the order of 500 volts between the cathode I8 and the end 35 of the cylindrical chamber I2. The heated cathode for emitting electrons, the focusing electrode 24', and the polarized end 36 of the chamber I2 cooperate to project the beam of electrons through chamber I2 toward the exit opening in the end 31 thereof.

If desired, the cylindrical chamber I2 may be grounded, as illustrated in Fig. 1 at 43.

A collector polarizing source such as a battery 44 is .provided for maintaining the collector electrode 33 at a relatively small positive potential with respect to the cylindrical chamber I2 and ground, a potential of the order of 25 volts being adequate for this purpose. The negative terminal of the battery 44 is connected to the cylindrical chamber I2 and to ground at a junction 45. and its positive terminal is connected through an impedance such as a resistor 46 to the collector electrode 33.

A wave analyzer 41 arranged to provide a meas` urement of alternating energy at any frequency selected by adjustment of a variable frequency control is coupled at one termin al to the collector electrode 33 of the device I I by a coupling capacitor 48, the other input terminal of the wave analyzer being connected to junction 45. The wave analyzer 41 may be such an instrument as the General radio wave analyzer, model 'B6-A. Alternatively, it may be such an instrument as is shown in U. S. Patent 1,994,232, issued March 12, 1945, to O; H. Schuck, Jr., for Wave analyzer.

In the operation of the electron discharge device illustrated in Fig. 1, the cathode I8, which may have a planar or slightly concave electronemitting surface is heated by the heater electrode 23 and is made to emit electrons, which are attracted toward the electron entrance opening in the wall 36 to the chamber I2. The focusing electrode 24 cooperates with the cathode I8 to produce predetermined voltage gradients in the vicinity of the cathode I8 such that the electrons emitted thereby are projected in a concentrated electron beam or stream along a predetermined axis, which may be the axis of the cylindrical chamber I2.

Those electrons in the beam projected through the chamber I2 which are near the axis of the chamber are permitted to pass through the exit opening 31 and to be received by the collector electrode 33. On the other hand, those electrons which are spaced appreciably from the axis of the chamber I2 are arrested by the end wall 31 of the device, and thus are prevented from' being received by the collector electrode 33.

The electric current which circulates through the cathode I8, the collector electrode 33, the impedance 46, and energy sources or batteries 4d and 42, thus increases or decreases accordingly as the electrons in the beam projected by the electron gun I3 are closely concentrated or scattered in the region near the exit through the Wall 31 of the chamber I2.

Since as pointed out above, the scattering of the electrons from the axis of the electron beam is sharply dependent upon the number oi.' positive ions in the beam, and since slight variations in the number of positive lons in the beam occur at a frequency substantially proportional to the gas pressure therein, the frequency distribution of variations in the beam scattering is indicative of the gas pressure within the space through which the electron beam is projected. Accordingly, the variations in the electric current through4 the collector electrode 33, and thus of the voltage produced across the impedance I6, may be frequency analyzed to determine the gas pressure in the device Il, and thus the gas pressure in the system in which the device ii is connected by conduit I5.

In Fig. 2 is shown a graph including a family of three curves, each of which represents the distribution of electrical output energy vs. frequency corresponding to a selected pressure of the gas within the device I I. Curve I, representing a relatively poor vacuum; or high gas pressure, has its maximum energy content at a relatively high audio frequency fi, and has lower intensity at frequencies above and below f1. Curve II, representing a better vacuum, which might be characterized by a gas pressure of the order of '1 millimeter of mercury, for example, contains maximum energy at an appreciably lower frequency f2, again having a wide range of frequency components above and below this value at which the output is appreciably reduced. Curve IH illustrates an energy vs. frequency distribution having a peak at an extremely low fre-Y quency fa, corresponding to an extremely high vacuum, as., for example, a gas pressure of the order of 10-9 millimeter of mercury. The frequency of the maximum alternating current energy derived from the electron beam is substantially proportional to the gas pressure.

In the apparatus shown in Fig. 1, the substantially unvarying total electron current in the beam is divided into two parts, the rst part being a selected middle or central portion of the electron stream which is permitted to pass through the opening in the end wall 3l, and the other part being the outer part of the electron stream, which is arrested by the end wall 3l and thus prevented from reaching the collector electrode 33. With the compactness modulation of the electron beam, the current through the collector electrode 33 increases as the beam compactness increases and decreases as the `beam compactness decreases. rI'he electron current produced by the collection of the arrested electrons upon the end wall 3l varies conversely, increasing with a decrease of electron beam compactness and decreasing with an increase of electron beam compactness. It will be readily apparent, therefore, that the electron current due to collection of electrons by the end wall 3l is modulated in equal amplitude and reversed phase with respect to the current through the collector electrode. If desired, therefore, a wave analyzer could be coupled to an impedance inserted in series with the connection of the positive terminal of source 42 to the cylindrical chamber i2. This wave analyzer would yield data corresponding exactly with the data obtained through use of wave analyzer 4l connected as shown. Thus, it is optional whether the frequency distribution of energy due to the compactness modulation of the beam be determined by measuring the electron current through a predetermined middle part of the beam or by measuring the electron current outside this middle part of the beam.

It is not essential to the present invention that the electron discharge device i i be constructed in the form illustrated in Fig. 1. Various types oi' vacuum tube structures, such as that oi' the Klystron or velocity modulation tube illustrated in U. S. Patent 2,242,275 vto Russell H. Varian, could be employed as the electron discharge device ll the conduit I5 then being replaced by the tubular connection through which the tube is connected to a vacuum pump during manufacture. Since many types of vacuum tubes of the above class incorporate electron beam projecting devices as well as electron collector electrodes, such vacuum tubes are inherently suited to be connected in the same manner as the electron discharge device Il in Fig. 1, so that no auxiliary electron discharge device need be employed for measurement of gas pressure in the Klystron. Certain other types of vacuum' tubes, also employing directed electron beams, are adapted to be operated while in the evacuation process, and to give output vs. frequency data indicating their own gas pressures.

Since many changes could be made in the above construction and many apparently widely different-embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawingshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Gas pressure responsive apparatus comprising: an electron discharge device having a vchamber including an electron entrance and an electron exit therein, means for directing a concentrated-stream of electrons through said electron entrance toward said electron exit, and a collector electrode positioned beyond said exit for receiving electrons passing therethrough; means including a load impedance connected between said collector electrode and said electron stream directing means for providing a current flow path; and means coupled to said load impedance for determining the variation of amplitude with frequency of alternating components of current through said collector electrode.

2. Apparatus as defined in claim 1, wherein said means including a load impedance for providing a current iiow path also includes means for raising the potential of said collector electrode above the potential of said chamber.

3. Apparatus for determining the gas presm sure within an evacuated space, lcomprising means for projecting a beam of electrons through said space, a collector electrode positioned in the path of said electron beam for receiving said electrons, circuit means coupling said collector electrode to said projecting means for conducting current therebetween, and frequency-responsive means coupled to said collector electrode and said beam projecting means for determining the frequency distribution of variations of current in said electrode.

4. Apparatus as defined in claim 3, further including means'positioned adjacent said collector electrode and between said projecting means and said collector electrode for arresting electrons which are divergent from said path.

5. High-vacuum gas pressure measuring apparatus comprising means for projecting a beam of electrons through an evacuated space, means for selectively rejecting electrons beyond a central portion of said beam and receiving electrons in said centralportion of said beam, and

frequency-selectivo amplitude-responsive means coupled to said last-named means and said beam projecting means for analyzing the frequency distribution of voltage variations of said electrode to determine the gas pressure in said space.

6. High-vacuum' gas pressure measuring apparatus comprising means for projecting a beam asuma of electrons through an evacuated space, means f for separately receiving electrons within a central portion of said beam and those electrons in the outer portion of said beam, and frequencyselective amplitude-responsive means coupled to said last-named means and said beam projecting means for analyzing the frequency distribution' 8. The method of determining the gas pressure within an evacuated chamber, comprising pro- Jecting a beam of electrons along an axis in said chamber, arresting those electrons which are separated from said axis by more than a predetermined distance. and measuring the frequency distribution of alternating components of variation of the non-arrested electrons.

9. The method of measuring gas pressure within an evacuated space comprising directing a beam of electrons through said space and producing ions therein at a rate determined by the gas pressure in said-space, the compactness of said beam being modulated as said ions are produced, and measuring the alternating energy versus frequency characteristics of the electrons in a predetermined portion of said beam.

RUSSELL H. VARIAN.

REFERENCES ITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,649,016 Buckley Nov. 15, 1927 2,081,429 Gaede May 25, 1937 2,242,275 Varian May 20, 1941 Cert-iicate of Correction March 28, 1950 Patent No. 2,501,702

RUSSELL H. VARAN 1t is hereby certified that error appears in the print numbered patent requiring correction as follows'.

Column l, line 27, before the syllable elec insert the Words electron beam becomes as great as the number of;

ers Patent should be read with this correction the Patent Ofliee.

and that the said Lett same may eonorm to the record of the case 1n d sealed this 20th day of June, A. D. 1950.

Signed an ed speecation of the above therein that the [SEAL] THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,501,702 March 28, 1950 RUSSELL H. VARIAN It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column l, line 27, before the syllable elec insert the Words electron beam becomes as great afs the number of;

nt should be read With this correction therein that the and that the said Letters Pate d of the case in the Patent Office.

same may conform to the recor Signed and sealed this 20th day of June, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Asse'stfm Uommz'ssz'meer of Patents.

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